In plastic injection molding processes, a thermoplastic or thermoset molding compound is first heated to plasticity in an injection cylinder at controlled temperature. Afterwards, the plasticized compound is forced from the cylinder through a nozzle by means of pressure generated within the cylinder. On emerging from the nozzle, the plasticized compound is conveyed through a hole in a mold plate, usually stationary, by means of a sprue bushing (sprue for short) into the runner system or gate of the mold cavity. Once in the cavity, the resin, assuming the shape of the cavity, is cooled to the point where it is sufficiently solidified to retain the desired cavity shape. The mold is then opened, and the part ejected or otherwise removed. The entire process is usually automated, with the clamping of the mold parts prior to injection and unclamping for part ejection or removal after cooling taking place under the control of a microprocessor or other form of automated controller. Machines are rated by the number of ounces they will inject per piston or screw stroke and by the square inches of working area that can be clamped against injection pressure. Consequently, a machine can be used to fabricate a variety of different sized parts up to its maximum capacity. This is accomplished by using, among other things, plates of suitable thickness for the part at hand; large working areas and high injection pressures calling for robust mold plates to mount and sustain the large stresses created during the molding cycle. Plates used to make small parts, on the other hand, can be scaled down in size in accordance with the reduced stresses generated while molding them. Parts between the largest and smallest obviously require intermediate sized plates, and the industry has adopted standard plate thicknesses to promote productivity.
For best process performance regardless of part size, it is known to be beneficial to keep the temperature of the plasticized compound more or less constant throughout its travel to the cavity. This reduces process problems associated with material degradation due to thermal variability, improves yield by decreasing scrap losses, and increases machine production time by reducing down time due to freeze offs.
However, while standardized in many respects, particularly with respect to mold base or plate thicknesses, present molding machinery does not always provide for precise temperature control to take advantage of its beneficial effects. Indeed, much of the available machinery is stiff run employing cold sprue bushings which allow the temperature of the resin to be poorly controlled from the time it leaves the nozzle until it reaches a zone in the mold where temperature control is reacquired with, for example, internal heating channels in the mold.
More sophisticated practice, apparently not yet universally accepted, recognizes the advantages of controlling temperature by employing hot sprue bushings to convey material from the nozzle to the cavity gate, often times through the fixed mold plate, sometimes referred to as the "A" plate or base. A variety of approaches for providing heat in these hot sprue bushings have been used. Among these are the use of resistive heating elements and heat pipes such as those described in U.S. Pat. No. 4,034,952 entitled "HOT PLASTIC INJECTION BUSHING" issued on Jul. 12, 1977. In the latter case, the heat pipes are used to transfer heat from electrically powered heater bands located at the nozzle end of the sprue bushing to regions along the bore near the tip.
U.S. Pat. No. 5,312,824 entitled "ADJUSTABLE HOT SPRUE BUSHING" issued on May 25, 1993, and assigned to the same assignee as the present application, describes a hot sprue bushing of adjustable drop length so that it can be used in injection molding machines with a full range of different mold plate thicknesses to transport plasticized material from injection nozzles to mold cavities while controlling the temperature of the plasticized material. This hot sprue bushing comprises an elongate body having a head, a stem and a tip; the head has a reference seating surface which faces the tip. The elongate body has a through bore passing from end to end through the head, stem and tip of the body for transporting plasticized material through the sprue bushing from the head to the tip thereof. The body also has at least one non-through bore extending alongside the through bore, in heat conducting relationship with respect thereto; the or each non-through bore extends from the head through the stem and into the tip but is shorter than the elongate body. The or each non-through bore receives an electrically powered heating cartridge for controlling the temperature of the plasticized material as it travels along the through bore. In a preferred form of this hot sprue bushing, the body contains yet another bore which receives and positions a thermocouple substantially at the tip of the elongate body for controlling the temperature of the through bore over its full length.
Adjustment of the drop length of the hot sprue bushing is effected by means of a changing means manually positionable against the reference seating surface of the head and between this reference seating surface and the tip for shortening the initial drop length of the elongate body. This changing means has a mold plate seating surface selectively locatable over a predetermined range of distances between the reference seating surface and the tip to change the drop length of the hot sprue bushing, the changed drop length of the hot sprue bushing corresponding to the distance between the mold plate seating surface of the changing means and the tip. The hot sprue bushing may also have a cap which removably attaches to the head and has a recess having a shape complementary to that of standard injection nozzle shapes.
The adjustable hot sprue bushing of U.S. Pat. No. 5,213,824 gives good control of the temperature of the plasticized materials passing therethrough and is in commercial use. However, because the body of this hot sprue bushing needs to accommodate the through bore for the plasticized material, (typically) four bores for the heater cartridges and the bore for the thermocouple, the body needs to of substantial cross-section, and thus has a high thermal mass. This high thermal mass requires substantial heat input from the heater cartridges to maintain the proper operating temperature in commercial use, and thus the heater cartridges are subjected to a heavy duty cycle, which reduces the working life of the heater cartridges. Such cartridge failure results in substantial down time of the of the injection molding machine, since it is necessary to disassemble the hot sprue bushing in order to replace the failed heater cartridge. It has also been found that, in commercial use, the cap of the hot sprue bushing is easily damaged and detached from the head. Finally, in the hot sprue bushing of U.S. Pat. No. 5,213,824, the wiring for the heater cartridges emerges from the bushing on a bracket secured to the cap of the bushing, and thus immediately adjacent the injection nozzle, in which location the wiring is somewhat susceptible to damage.
It is an object of the present invention to provide a hot sprue bushing which allows control of the temperature of the plasticized materials passing therethrough similar to that of the bushing described in the aforementioned U.S. Pat. No. 5,213,824, but having a lower thermal mass so that the heater of the bushing can be operated at a lower duty cycle, and thus with an improved working lifetime, as compared with the heater cartridges used in this patent.
It is also an object of the present invention to achieve such a bushing with lower thermal mass which still permits location of a thermocouple adjacent the tip of the body for controlling the temperature of the through bore over its full length.
It is a further object of the present invention to achieve the aforementioned objects while still allowing adjustment of the drop length of the hot sprue bushing.
It is a further object of preferred embodiments of the present invention to eliminate the vulnerable cap of the hot sprue bushing of U.S. Pat. No. 5,213,824.
It is a further object of preferred embodiments of the present invention to allow the wiring for the hot sprue bushing to emerge from the injection molding machine at a point spaced from the bushing, thereby rendering this wiring less susceptible to damage.
Other objects of the invention will be apparent and will appear hereinafter in the following detailed description when read in connection with the drawings.